XAFS study on liquid selenium under high pressure

Using large-volume presses, it is now possible to measure X-ray absorption spectra under a wide range of pressure-temperature conditions. X-ray absorption fine structure (XAFS) has successfully been measured at the K edge of crystalline and liquid Se at temperatures up to 1023 K and pressures of about 5 and 8 GPa, and at temperatures up to 1173 K at about 2.5 GPa. Crystalline Se consists of infinite chain molecules. At atmospheric pressure, it is known that the chain structure of Se is largely preserved upon melting. The temperature dependence of the extended X-ray absorption fine structure (EXAFS) at 2.5 GPa indicates that twofold covalent bonds remain upon melting, as at atmospheric pressure. On the other hand, the decrease of EXAFS oscillation upon melting at 8 GPa is larger than that expected from the temperature dependence of EXAFS in the crystalline state, indicating that the covalent bonds are modified in the liquid state. The change of structure of liquid Se is discussed in relation to a proposed semiconductor-metal transition under high pressure.